1. Field of the Invention
The present invention relates to a computer generated hologram, an exposure apparatus, and a device fabrication method.
2. Description of the Related Art
A projection exposure apparatus has conventionally been employed to fabricate, for example, a micropatterned semiconductor device such as a semiconductor memory or logic circuit by using photolithography. The projection exposure apparatus projects and transfers a circuit pattern formed on a reticle (mask) onto, for example, a wafer via a projection optical system.
A resolution R of the projection exposure apparatus is given by:R=k1×(λ/NA)  (1)where λ is the exposure light wavelength, NA is the numerical aperture of the projection optical system, and k1 is a process constant determined by, for example, a development process.
The shorter the exposure light wavelength or the higher the NA of the projection optical system, the better the resolution. However, it is difficult to further shorten the current exposure light wavelength because the transmittance of a glass material decreases as the exposure light wavelength shortens. It is also difficult to increase the NA of the projection optical system because the depth of focus decreases in inverse proportion to the NA of the projection optical system, and because it is hard to design and manufacture lenses to form a high-NA projection optical system.
Under the circumstances, there have been proposed resolution enhanced technologies (RETs) of improving the resolution by decreasing the process constant k1. One of these RETs is the so-called modified illumination method (or oblique illumination method).
The modified illumination method generally inserts an aperture stop, which has a light-shielding plate on the optical axis of an optical system, in the vicinity of the exit surface of an optical integrator which forms a uniform surface light source, thereby obliquely irradiating a reticle with exposure light. The modified illumination method includes, for example, an annular illumination method and quadrupole illumination method that are different in the aperture shape of an aperture stop (i.e., the shape of the light intensity distribution). There has also been proposed another modified illumination method which uses a computer generated hologram (CGH) in place of an aperture stop, in order to improve the use efficiency (illumination efficiency) of exposure light.
Along with an increase in the NA of the projection optical system, a polarized illumination method which controls the polarization state of exposure light is also becoming necessary to increase the resolution of the projection exposure apparatus. The polarized illumination method basically illuminates a reticle with not P-polarized light but S-polarized light alone, which has a concentric component with respect to the optical axis.
In recent years, there has been proposed a technique which attains both the modified illumination method (the formation of a light intensity distribution having a desired shape (e.g., a quadrupolar shape)) and the polarized illumination method (polarization state control).
For example, Japanese Patent Laid-Open No. 2006-196715 discloses a technique which attains both the modified illumination method and polarized illumination method using one element. Japanese Patent Laid-Open No. 2006-196715 controls the shape (reconstructed image) of the light intensity distribution using a CGH, and controls the polarization state using structural birefringence. More specifically, this technique forms one CGH by parallelly arranging a plurality of CGHs (to be referred to as “sub-CGHs” hereinafter) corresponding to light beams in the same polarization direction, and applies structural birefringence corresponding to the polarization direction to each sub-CGH.
Japanese Patent Laid-Open No. 2006-49902 selectively uses a desired polarization mode by adopting a polarization controller as a unit for controlling polarization modes applied to a sub-CGH.
Japanese Patent Laid-Open No. 2006-5319 discloses a technique which can control the balance among four poles of a quadrupolar light intensity distribution representatively formed by the modified illumination method and polarized illumination method. More specifically, Japanese Patent Laid-Open No. 2006-5319 forms sub-CGHs by dividing a CGH into four, and changes the intensity distribution of incident light, thereby making it possible to change the pole balance of a reconstructed image obtained by the CGH.
However, the prior arts form sub-CGHs by dividing one CGH into a plurality of CGHs, so an illuminance variation occurs in a reconstructed image if an optical integrator cannot sufficiently correct the intensity distribution of incident light (for example, if light enters only some of these CGHs).
When a plurality of sub-CGHs are combined, unnecessary diffracted light is generated due to structural discontinuity that occurs at the boundary between the sub-CGHs, resulting in deterioration in a reconstructed image obtained by the CGH. Structural discontinuity that occurs at the boundary between the sub-CGHs can be eliminated by improving the design of a computer generated hologram, but this poses another problem that the design cost increases enormously.
When polarization modes are selectively used by a polarization controller, the use efficiency (illumination efficiency) of light (exposure light) from an exposure light source decreases significantly (i.e., a loss in light amount increases).